The present invention relates to power driven machinery, such as compressors, and, more particularly, to centrifugal compressors.
Due to the power that drives them and the need for durability, machines such as compressors tend to be heavy and bulky. The bulk involved requires that considerable space be dedicated to machines such as compressors, and it has long been desired to reduce their space requirements, not only for more advantageous use of space in buildings and the like, but also to reduce the size of the machines for purposes of shipping, maintenance, repair, etc. In the case of a compressor, for example, the output shaft from an electric motor, internal combustion engine, or the like is coupled to a drive shaft of the compressor. The drive shaft transmits power from the power source through an arrangement of gears to another shaft, on which an impeller is mounted. Very often, where more than one stage of compression is necessary, one or more additional impellers are mounted on still other shafts. All of this tends to add to the size of the compressor.
Furthermore, the input shaft to the compressor must be precisely aligned with the output shaft from the motor or the like to avoid excessive vibration, premature bearing failure and other problems. In addition, the gears tend to be noisy, and there are a number of places at which high-pressure gas from gas handling portions of the compressor can leak into areas containing the gears and other drive elements. In addition, a large seal around the input shaft has been required and, in some cases, notches in the seal, designed to prevent seals from rotating, that can lead to seal distortion as the temperature rises. Moreover, many step up gear drives employ a single stage of step up, thereby making the pitch line velocities of the gears and the gear noise level higher, and gear efficiency lower. In addition, gear size is unnecessarily large. With a single stage of gear step up, the drive shaft and the impeller shaft [input and output] must be offset, requiring more expensive machining and assembly and the windage loss of the gears traveling through the gas in the drive compartment is higher than necessary because the gear diameter is greater and the gear velocity is higher than necessary. The power windage loss from a gear is usually proportional to the fifth power of the diameter. With many shaft bearings, there tends to be some movement between bearing surfaces and the shaft such that the shaft is slightly eccentric to the bearings, which in turn increases the likelihood of significant leakage of the impeller cavity to the gear cavity.
As with all rotary machinery, particularly machinery that rotates at a high speed, vibration is an important consideration, and vibration is likely to be excessive if the critical speed of the rotor is below the operating speed of the rotor, the critical speed being the speed at which vibration begins. Each impeller develops pressure from the inside to the outside and, as a result, each impeller has a thrust force developed toward the inlet side. These thrust forces must be borne by some portion of the compressor.
By the present invention, a centrifugal compressor for air or other gases is provided which is compact and lightweight compared with known centrifugal compressors of similar capacity. In addition, the centrifugal gas compressor according to the present invention has numerous other improved characteristics, such as reduced noise and vibration, increased ease of installation, improved sealing between the gear cavity and the compressor cavity, as well as smaller and more efficient sealing between the compressor and the atmosphere. Furthermore, aspects of the compressor according to the present invention result in lower costs for machining and assembly than known compressors.
The foregoing and other advantages result from the construction of the compressor of the present invention. Misalignment of a coupling of the drive shaft of the motor, engine or the like to the drive shaft of the compressor is accommodated by the structure of the compressor shaft, including the thinness and flexibility of the compressor input shaft. The compressor input shaft is a thin quill shaft positioned within a hollow shaft so that most of the length of the hollow shaft overlaps the quill shaft. The quill shaft is fixed to the hollow shaft only at the internal ends of the quill shaft and the hollow shaft, thereby reducing the overall length of the compressor input shaft system compared to the input shaft systems of compressors in which the thin quill shaft is mounted external to the compressor. Thus, according to the present invention, the full length of the quill shaft between the coupling to the motor at one end and the attachment to the hollow shaft at the other end is available to accommodate strain and misalignment, but without increasing the length of the compressor input shaft system by the entire length of the compressor input shaft and without adversely affecting the performance of the hollow shaft.
The diameter of the thin quill shaft can be minimized because no keyways are used to connect the shaft to the coupling hub. Keyways inherently experience increased stress concentrations relative to the rest of a shaft. Therefore, shafts having keyways must be made sufficiently large in diameter to withstand the higher stresses of the keyways. A hub is mounted by friction on the outer end of the thin quill shaft, the hub being connected by a flexible coupling with the motor drive shaft. The inner end of the thin quill shaft is secured by friction in the inner end of the hollow shaft. Since the drive shaft of the compressor according to the present invention is a round shaft without keyways, the drive shaft has no increased stress concentrations in some areas. As a result, the diameter of the shaft can be made smaller. This reduces weight and permits the use of a smaller shaft seal having lower rubbing speeds, lower costs, and less friction loss.
In addition to accommodating misalignment of the coupling, the small diameter of the quill shaft allows the shaft seal between the gas in the compressor and the atmosphere to be minimized, due to the smaller diameter of the seal. The smaller diameter of the seal reduces the speed at which surfaces of the seal rub on one another and thereby reduces wear on the seal and the possibility of leakage.
Of the parts of the seal that rub on one another, it is preferred that one is carbon and that the other is silicone carbide. These materials operate together with a low coefficient of friction to reduce wear. Furthermore, the carbon seal is devoid of notches and other irregularities and, therefore, is better able to maintain its true flatness without distortion. Notches are commonly used in seals to receive complementary formations and thereby prevent the seals from rotating due to the friction between the mechanical parts. Upon the heating due to friction, the areas of the seals around the notches expand differently from other areas of the seals, thereby causing distortion of the seals. In contrast, the compressor according to the present invention includes a circular spring finger to grip the carbon seal part and thereby prevent it from rotating due to friction.
Two stages of gear ratio step up are used in the compressor according to the present invention. As a result, the pitch line velocities of the gears are much lower than the pitch line velocities of a single stage step up gear. This makes for better efficiency and a lower noise level. Furthermore, the use of a two-stage gear ratio step up enables the compressor drive shaft and the compressor impeller shaft to be coaxial, rather than offset from one another, as in a single stage gear ratio step up. The coaxial arrangement reduces the cost of machining and assembly. Furthermore, the diameters of the gears in the two-stage gear ratio step up arrangement are smaller than in a single stage arrangement, and the gear velocities are lower. As a result, the power loss due to the windage of the gears travelling through the gas surrounding the gears is much lower. The power windage loss from a gear is usually proportional to the diameter of the gear to the fifth power. Moreover, since the two-stage gear arrangement enables the drive shaft and the impeller shaft to be coaxial, the overall diameter of the compressor casing can be much smaller than would be the case where one shaft is offset from other shafts by the center distance between the gears.
The two-stage gearing of the compressor according to the present invention employs three cluster gears spaced by 120xc2x0 from one another around the axis of the drive shaft. A center gear is mounted on the input shaft of the compressor and is supported by the three cluster gears. The use of the three cluster gears rather than a single offset gear means that the center gear can handle three times as much torque. As a result, the center gear need not be as large as in other arrangements, and so the size and weight of the compressor is further reduced.
The impeller shaft and all of the rotating shafts of the gears are mounted on angular contact ball bearings. These bearings are spring loaded so that they are always maintained precisely concentric with the shaft. As a result, there is less movement of the shaft eccentric to the bearings, and this reduces the possibility of much leakage of gas from the impeller cavity of the compressor into the gear cavity. The impeller shaft is supported by bearings at three points along the line of the shaft: the bearing at one end of the impeller shaft comprises the center gear on the impeller shaft being supported by the three cluster gears; a bearing adjacent the impellers directly supports the load of the impeller on the impeller shaft; and an intermediate bearing supports the middle of the impeller shaft and takes any unwanted thrust load on the shaft. In a shaft having bearings at three points along its length, it is almost impossible to get excellent results if the shaft is stiff between all three bearings. Accordingly, in the compressor according to the present invention, the portion of the impeller shaft between the cluster gears and the intermediate bearing is a small diameter quill shaft. This portion of the shaft permits relative transverse deflection of the center gear with respect to the other two bearings, which is an important factor in making the operation of a three-bearing shaft successful.
Where two impellers are used, they are mounted together, back to back, on the impeller shaft and forced tightly against one another. This arrangement permits the distance from one stage of compressor to another to be a minimum. It also permits both impellers to be positioned close to a bearing, so that both impellers are well supported, and the critical speed of the rotor can be well above the operating speed. This reduces the possibility of vibrational damage. By arranging the impellers back to back, the thrust forces tending to move each impeller toward its inlet side counteract each other, so that the net thrust from the impellers is minimized. This also enables the use of a smaller balance piston seal between the stages of the two impellers.
Inside the casing of the compressor is an internal gear housing which provides a barrier against sound emissions from the gears to the outside of the compressor. This double wall installation eliminates the transmission of much of the gear noise to the outside.